As a method for remarkably improving the durability of a spring, shot peening is an indispensable process for a high-strength spring, especially for a suspension spring used in automobiles or a valve spring used in engines.
In the shot peening process, a number of small particles are projected onto the surface of the target object. This process is apparently the same as the shot blast, a process that is performed to make the surface clean by removing burrs (or projections) resulting from cutting or forming work or scales (i.e. a hard oxide layer) resulting from a heat treatment. However, the two processes significantly differ from each other in respect to the strength and other conditions; for shot peening, the conditions are determined to cause a plastic deformation only on the surface of the spring so that a compressive stress remains on the surface.
The main purpose of shot-peening a spring is to generate beforehand a compressive residual stress within the surface of the spring so that the load stress working on the spring when it is in service is reduced by an amount equal to the residual stress. For this purpose, various shot peening methods have been developed to attain as high a residual stress as possible.
For example, the Japanese Examined Patent Publication No. S48-20969 discloses a technique in which a piece of spring steel having a sorbite structure is shot-peened under a warm environment with a temperature of 200 to 400° C. after the quenching and tempering processes.
The Japanese Unexamined Patent Publication No. S58-213825 discloses a technique in which the shot peening is performed while the temperature of the spring is within the range from 150 to 350° C. in the course of the cooling process after the temper-heating process.
The Japanese Unexamined Patent Publication No. H05-140643 discloses a technique for generating an adequate level of compressive residual stress, in which a piece of steel having a predetermined composition undergoes a warm shot peening process while the temperature is maintained within the range from 150 to 300° C. after the thermal refining process, i.e. the quenching and tempering processes.
The techniques disclosed in the aforementioned three publications were first developed in the days when springs were used under low levels of working stress. Such past techniques could not always meet the performance requirements for the latest springs that were put in service under much higher levels of working stress.
To solve such a problem, the present invention intends to provide a method for manufacturing a high-strength spring, which is capable of generating a higher level of compressive residual stress than that generated by conventional methods.